A Universal Serial Bus (USB) standard has been developed which allows up to 127 peripheral devices such as printers, scanners, keyboards, modems, cameras and storage devices to be attached to a host, usually a personal computer (PC), through a 4-wire bus. These devices can be connected to the PC either directly or via hubs which provide additional connections. USB has the advantage that connection of different types of devices becomes standardised. Furthermore, a device can be connected while the PC is switched on and while other devices are in use. USB can support data rates as high as 480 Mbps. An overview of USB may be found in “USB Hardware & Software” by J. Garney et al. (Anna Books, 1998) [ISBN 0-929392-3-X] and a current version of the USB specification is available at www.usb.org or from USB-IF Administration, 5440 SW Westgate Drive, Suite 217, Portland, Oreg. 97221 USA.
Mobile telephone handsets are being introduced which support USB. A USB interface allows them to be connected to a PC or a peripheral device. In this way, a handset may serve as a peripheral device or a host. Although such a handset will enjoy many of the advantages of USB, there are, however, some drawbacks. Simple peripheral devices for connecting to the handset are made unnecessarily complex and expensive since they themselves must support USB. Furthermore, once connected, these devices draw a high current from the handset, which has limited power resources.
Mobile telephone handsets are known which have simple serial communications interfaces based on the RS-232 standard and which use single-ended data transmission. This allows them to be connected to peripheral devices such as keyboards for entering text for short message service (SMS) and e-mail. These types of interfaces draw much less current and so are better suited to low power applications.
To overcome the drawbacks of having a USB interface, both types of interface may be included in a single handset. However, this is not practical for a small handset. Furthermore, this defeats the object of providing a standardised connector.
The present invention seeks to provide a solution to this problem.